mondrake
--- a/core/lib/Drupal/Component/Plugin/Discovery/AttributeClassDiscovery.php
+++ b/core/lib/Drupal/Component/Plugin/Discovery/AttributeClassDiscovery.php
  protected function parseClass(string $class, \SplFileInfo $fileinfo): array {
    // @todo Consider performance improvements over using reflection.
    // @see https://www.drupal.org/project/drupal/issues/3395260.
-    $reflection_class = new \ReflectionClass($class);
+    try {
+      $reflection_class = new \ReflectionClass($class);
+    }
+    catch (\Throwable $t) {
+      // If reflection failed on the class, it could be due to the class
+      // implementing or extending from interfaces/classes that are not
+      // available to the classloader. This can happen in contrib when a class
+      // in a module extends from a class in another module that is not
+      // installed. In that case, just skip the plugin.
+      return ['id' => NULL, 'content' => NULL];